A natural gas produced from a wellhead is subjected to liquefaction pretreatment for separating impurities, and then cooled to be liquefied, and shipped as a liquidized natural gas (LNG).
Some natural gases contain, as an impurity, a non-hydrocarbon gas, such as a carbon dioxide gas (CO2 gas) or a nitrogen gas (N2 gas), in a relatively large amount. When this kind of natural gas is treated, the non-hydrocarbon gas is discharged in a large amount in the liquefaction pretreatment.
As a method of utilizing the non-hydrocarbon gas discharged in the liquefaction pretreatment of the natural gas and reducing the emission of the non-hydrocarbon gas into the atmosphere, there is given an example in which the non-hydrocarbon gas is utilized as an injection gas in enhanced oil recovery (EOR) or enhanced gas recovery (EGR) by being injected into an oil well or a gas well. In addition, when the non-hydrocarbon gas is a CO2 gas, an example in which the CO2 gas is subjected to carbon dioxide capture and storage (CCS) in the ground and an example in which the CO2 gas is utilized as a raw material in a urea production plant are also conceived.
Meanwhile, in the liquefaction pretreatment, as a non-hydrocarbon gas separation device configured to separate a non-hydrocarbon gas from a natural gas, there is given a non-hydrocarbon gas separation device utilizing a separation membrane capable of performing gas separation. This kind of non-hydrocarbon gas separation device is hardly accompanied by a phase change at the time of gas separation, and is configured to perform gas separation by using, as a drive energy, a difference in pressure (difference in partial pressure) of a gas to be separated before and after its permeation through the separation membrane, and utilizing a difference in gas passage speed through the separation membrane.
The non-hydrocarbon gas separation device utilizing a separation membrane has the advantages of high energy saving performance and easy handleability. Meanwhile, when the natural gas containing a non-hydrocarbon gas in a large amount is to be treated, equipment and piping constructing the non-hydrocarbon gas separation device tend to be increased in size, and there is a problem of an increase in facility cost.
In addition, when non-hydrocarbon gas recovery, such as EOR or CCS described above, is performed, the non-hydrocarbon gas needs to be sent after its pressure is increased to a pressure receivable in a recovery facility in some cases. In the case of the natural gas containing a non-hydrocarbon gas in a large amount, there is another problem in that power to send the non-hydrocarbon gas to the recovery facility is increased.
In this connection, in Patent Literature 1, there is a description of a carbon dioxide separation system in which a primary carbon dioxide separation device equipped with a zeolite membrane for carbon dioxide separation and a secondary carbon dioxide separation device that employs an amine absorption method or a pressure swing adsorption (PSA) method are connected in series. In addition, in Patent Literature 2, there is a description of a technology involving removing and recovering an acid gas from an absorbing chemical agent absorbing an acid gas, such as carbon dioxide, at a pressure of from 50 psi to 300 psi, and introducing a gaseous-phase stream containing the acid gas under the pressure to the intake of a compression device.
However, in the patent literatures, there is no description of a technology for efficiently sending the non-hydrocarbon gas while preventing an increase in size of the non-hydrocarbon gas separation device of a separation membrane mode.